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Large-aperture periodically poled Mg-doped LiNbO3 device using X-axis Czochralski-grown MgLN crystal was proposed to avoid a laser-beam distortion problem. Availability of periodic poling in 5-mm-thick MgLN and compatibility of wavelength-conversion characteristics in QPM-OPO were evaluated by comparing with conventional arrangement using Z-axis-grown crystal.
© 2014 Optical Society of America
1. Introduction
Nonlinear wavelength conversion by quasi-phase matching (QPM) technique [1, 2] can realize an efficient and various types of nonlinear wavelength conversion. Periodic inversion of spontaneous polarization by applying high-electric field [3, 4] enables us to fabricate ferroelectric QPM devices by using LiNbO3 (LN), LiTaO3 (LT), and KTiOPO4 (KTP). Wavelength conversion such as optical parametric oscillation (OPO) using the ferroelectric QPM devices pumped by conventional 1 µm laser source is suitable for realizing efficient and practical mid-infrared light sources, such as molecular spectroscopy, medical application and remote sensing [5]. In order to handle high-power/energy lasers by the QPM devices, increase of the QPM-device aperture is simple and effective method to avoid damages at both end face and device inside. Last several years, we have reported large-aperture (LA)-QPM devices using periodically poled Mg-doped congruent LN and LT (PPMgLN, PPMgLT) up to 10 mm thickness [6–8], which enabled us to handle a total OPO output energy > 0.5 J by 10 ns pulse duration at 10 Hz operation. Also in case of KTP, high-energy OPO was reported by using periodically poled 5-mm-thick Rb-doped KTP [9]. The LA-PPMgLN devices have been used for practical applications, such as ultra-short pulse generation [10, 11], broadband-tunable mid-infrared coherent-light source [12, 13], and highly sensitive THz light detection [14].
Recently, distortion of laser beam after passing the LA-PPMgLN device have become one of the problems in some applications, typically in the case of single-pass optical parametric generation (OPG) and amplification (OPA). The laser-beam distortion originates in the growth striation of widely used Z-axis Czochralski (CZ)-grown MgLN crystal, and become much severe not in waveguide-type or small-aperture device but in large-aperture device, because large-aperture beam includes many growth-striation lines. In this report, we present an improved LA-PPMgLN device, fabricated by using X-axis CZ-grown MgLN crystal. Fabrication and experimental characterization of the new PPMgLN arrangement are demonstrated to avoid the laser-beam distortion, and compared to the conventional PPMgLN arrangement.
2. Laser-beam distortion in PPMgLN device, origin and suppression
In most case at the use of current PPMgLN devices, a Z-cut chip is prepared for realizing periodic inversion by applying high-electric field along crystallographic Z-axis of MgLN, and the Z-cut chip is mainly obtained from a crystallographic Z-axis CZ-grown MgLN crystal because of an effective use of the MgLN crystal, as shown in Fig. 1(a). Also, many of the Z-cut PPMgLN device are used with the beam propagation direction parallel to crystallographic X-axis, because of the ease of fine periodic inversion [15]. In the conventional PPMgLN arrangement with Z-axis CZ-grown MgLN, growth striations occurred at the CZ-growth process exist in perpendicular plane against Z-axis of MgLN, which results in a beam distortion of PPMgLN device as Fig. 1(a).
Although a high-quality Z-axis CZ-grown MgLN crystal without the growth striations can simply suppress the distortion problem, it is not so easy to obtain an improved, stable and uniform MgLN crystal. Other choice to avoid this problem is to change the beam propagation axis parallel to the CZ-growth axis, which means the use of X-axis CZ-grown MgLN crystal in case of the X-axis propagation set up, as shown in Fig. 1(b). In this new PPMgLN arrangement with X-axis CZ-grown crystal, the CZ-growth striations exist in perpendicular plane against the beam-propagation axis ( = X-axis) and suppression of the laser-beam distortion can be expected compared to the conventional PPMgLN arrangement.
3. Evaluation of the laser-beam distortion by MgLN and PPMgLN
From our recent collaborating works, the laser-beam distortion and its related phenomena, such as efficiency distribution of nonlinear wavelength conversion along Z-axis, became severe especially in a single-pass application of the PPMgLN, such as OPG and OPA. The efficiency distribution along Z-axis originated by the CZ-growth striations is totally different from the efficiency distribution along Z-axis by the wedged structure of the periodic inversion in our current LA-PPMgLN device [8].
Beam-shape distortions of a He-Ne laser after propagation into various MgLN and PPMgLN along the X-axis direction were measured by using a high-resolution CCD camera (Ophir, SP620U, resolution 1600 x 1200, pixel size 4.4 µm x 4.4 µm) positioned 10 cm after the samples as shown in Fig. 2, and evaluated by using a correlation factor RZ of the propagated beam shape in Z-axis with perfect Gaussian shape. The original He-Ne beam (power ~1 mW, 1/e2 full width ~2.1 mm) has nearly Gaussian shape of RZ ~0.93 as shown in Fig. 3(a). Various Z-axis-grown MgLNs without QPM and with QPM structure of 40-mm length along X-axis were prepared from our previous stock [4, 6, 8] of different crystal boules for beam-distortion evaluation. After passing into the MgLNs, all of the propagated beams were more-or-less distorted along the growth striation of Y-axis at both Y- and Z-axis polarized He-Ne laser, and the beam distortion seemed to become severe in case of Y-polarized beam compared to Z-polarized beam. Here we note that the beam distortion of He-Ne laser could be measured even in a low power region < 50 µW and therefore the distortion does not occur by the photorefractive effects. Examples of the distorted beams in the Z-axis-grown MgLNs without and with QPM in case of Y-polarized beam are shown in Figs. 3(b) and 3(c), respectively. The two Z-axis-grown MgLNs were prepared from neighboring Z-cut chips in one Z-cut plate. Although the RZ for Figs. 3(b) and 3(c) could be evaluated to 0.62 and 0.79, respectively, the distorted-beam shape and the value of RZ easily changed by a slight shift of the beam position against the growth striation line, and the evaluated value of RZ from all MgLNs without and with QPM ranged widely from 0.29 to 0.84. After the evaluation of various MgLNs, we confirmed that the beam distortion occurs in all Z-axis-grown MgLNs without and with QPM and that an existence of QPM structure does not concern or increase the lined beam distortion as shown in Figs. 3(b) and 3(c) except for a fluctuation and non-uniformity of the growth striation in MgLN. Therefore, we concluded that the lined beam distortion originates in the Z-axis-grown MgLN itself.
Fig. 3 Measured He-Ne laser beam shapes, (a) Original, (b) after passing into MgLN from Z-grown without QPM, (c) MgLN from Z-grown with QPM, (d) MgLN from X-grown without QPM. The polarization of He-Ne laser was set along Y-axis of MgLN at (b)-(d). RZ presents a correlation factor of the propagated beam shape in Z-axis with a perfect Gaussian shape.
On the other hand, no severe beam distortion occurred in case of the X-axis-grown MgLN of 40-mm length without QPM structure as shown in Fig. 3(d), measured by a Y- polarized laser. The RZ was measured to 0.90, which is almost comparable with the original He-Ne beam. Therefore, we can expect a distortion-free LA-PPMgLN device by using a X-axis-grown MgLN, if we can fabricate high-quality periodic inversion in the X-axis-grown MgLN with the same quality as Z-axis-grown MgLN.
4. Fabrication and characterization of LA-PPMgLN using X-axis CZ-grown MgLN
QPM structure in MgLN can be realized by applying a periodic high-electric field more than a coercive field Ec to invert crystal polarization [3]. Although uniformity of the Ec is an important factor to obtain a uniform QPM structure along the beam-propagation axis, X-axis-grown MgLN for the suppression of the laser-beam distortion has a possibility of Ec dependence along X-axis, caused by the CZ-growth process. We tried to fabricate a QPM structure around 32 µm period (suitable for 1.064 µm-pumped OPO to generate mid-infrared light) in X-axis-grown 5-mm-thick (along Z-axis) MgLN Z-cut chip (maximum length of 40 mm along X-axis), and evaluated the obtained QPM structure by the Y-cut face etching and QPM-OPO wavelength characterization by 1.064 µm-pumped OPO experiments.
For a fabrication of LA-PPMgLN device, temperature-elevated field-poling technique is effective to decrease the Ec [4]. Figure 4 shows an obtained QPM structure with 32 µm period into the X-axis-grown 5-mm-thick MgLN Z-cut chip at a 16 kV high-voltage multi-pulse application in a 130°C oil-bath set up. These are almost same condition to our previous reports for Z-axis-grown MgLN [6, 8]. The obtained QPM structure in the X-axis-grown MgLN has a wedged shape that the periodic structure near + Z surface is almost merged with neighboring patterns and that penetration of inversion near -Z surface is insufficient, which is also same as of Z-axis-grown MgLN. There were no significant change or distribution of the Ec along X-axis (as long as 40 mm) for realizing the QPM structure. As a result, we confirmed the availability of LA-PPMgLN device from the X-axis-grown MgLN by using the almost same field-poling set up and conditions as conventional Z-axis-grown MgLN.
Fig. 4 Fabricated periodic structure in X-axis-grown, 5-mm-thick MgLN Z-cut chip. The QPM period is 32 µm.
For evaluating the QPM-OPO wavelength characteristics, OPO experiment pumped by a 1.064-µm Q-sw Nd:YAG laser (pulse duration = 10 ns, frep = 30 Hz) was demonstrated. Typical size of the PPMgLN device and area of the QPM poling were 40 mm x 16 mm x 5 mm (X, Y, and Z axis), and 38 mm x 5 mm (X, Y), respectively. Four different QPM period Λ of 32.2, 32.1, 31.8, 30.8 µm were prepared to check the OPO characteristics. The PPMgLN device was placed in a temperature-controlled oven at 25, 35, 45° C between input mirror and output coupler with a cavity length of 10 cm. The input mirror was a plane mirror with high reflectivity for both signal and idler waves, and high transmission for pump wave. The output coupler was also a plane mirror with partial reflection of ~40% for signal wave, high transmission for idler wave, and high reflection for pump wave. Wavelength range for signal and idler wave are 1.5 ~2.0 µm and 2.3 ~3.7 µm, and the wavelength of the signal wave was measured by a mid-infrared spectrometer (SOMA-OPTICS S2810, resolution ~30 nm) Further information of the OPO experiments is almost same as our previous reports [6, 8]. Figure 5 presents QPM-OPO wavelength characteristics depending on the QPM period and PPMgLN temperature. Empty marks show the measured results using the X-axis-grown PPMgLNs, and filled marks with interpolating lines mean the previous results using Z-axis-grown PPMgLNs for comparison. The QPM-OPO wavelength characteristics of the X-axis-grown PPMgLNs was well agreed with the results of Z-axis-grown PPMgLNs.
Figures 6(a) and 6(b) show He-Ne beam shapes after propagation into X-axis-grown PPMgLN with 40 mm length along X-axis direction by using (a) Y-axis polarized and (b) Z-axis polarized He-Ne laser, which showed no severe beam distortion in both polarizations. The RZ were 0.90 and 0.91 for Figs. 6(a) and 6(b), respectively. The propagated beam shapes could be evaluated to be almost same as that of X-axis-grown MgLN without QPM structure shown in Fig. 3(d), and comparable with the original He-Ne beam shown in Fig. 3(a).
Fig. 6 Measured He-Ne beam shapes after passing X-axis-grown PPMgLN. Propagation length in X-axis is 40 mm. He-Ne laser was polarized along (a) Y-axis and (b) Z-axis of original MgLN crystal.
These all results show that the X-axis-grown PPMgLN device can be compatibly used with the conventional Z-axis-grown PPMgLN and that the laser-beam distortion can be suppressed by changing the crystal growing axis.
5. Discussion
The growth striation in the conventional PPMgLN arrangement using Z-axis CZ-grown MgLN affects the beam quality of both transmitted and generated lights, and an effect of the laser-beam distortion become clear and severe in the large-aperture device, compared to waveguide-type or small-aperture device. Density of the growth-striation lines can be roughly estimated to be from several to several tens lines per mm, and effect of the laser-beam distortion considerably depends on the crystal quality, the beam position, and the laser-beam polarization. Because the LA-PPMgLN device for handling high-power/energy laser is also requested to realize high beam-quality wavelength conversion, the use of X-axis CZ-grown MgLN could be a simple and effective method to avoid the beam distortion problem until the realization of growth-striation free MgLN.
In conventional arrangement using Z-axis-grown MgLN, the growth striations affects as a line-shape distorting factor of the beam shape, which results in severe beam distortion in one direction along Y-axis of MgLN as Figs. 3(b) and 3(c). On the other hands, in new arrangement using X-axis-grown MgLN, the growth striations stays in almost perpendicular plane against the beam-propagation direction of X-axis, which cause slight fluctuation in whole beam shape as seen in Figs. 3(d), 6(a), and 6(b) compared to Fig. 3(a), and may give a small scattering loss. However, effect of the slight beam fluctuation in the X-axis-grown MgLN is enough low compared to the severe beam distortion in the Z-axis-grown MgLN.
Although we have reported about the OPO-output beam shape using a mid-infrared camera (Spiricon, Pyrocam NIR, resolution 124 x 124, pixel size 85 µm x 85 µm) in our previous report [8], we could not find a distorted OPO beam, such as Figs. 3(b) and 3(c). This may be caused by an insufficient resolution of the mid-infrared camera. Therefore, we want to evaluate an OPO-output beam shape again by using a higher-resolution mid-infrared camera.
6. Conclusion
For practical application of the LA-PPMgLN device, we presented an improved LA-PPMgLN device using a X-axis CZ-grown MgLN crystal. Fabrication of periodic structure and characterization of QPM-OPO were demonstrated using the X-axis CZ-grown crystal, and the beam distortion after passing a LA-PPMgLN device was evaluated for both Z-axis and X-axis grown crystal using a correlation factor with perfect Gaussian shape. As a result, we confirmed that the X-axis-grown PPMgLN device can be compatibly used with the conventional Z-axis-grown PPMgLN and that the laser-beam distortion can be suppressed by changing the crystal growing axis. Both high-power/energy handling ability and high beam-quality output have been requested in LA-PPMgLN device for recent applications such as ultra-short pulse generation and broadband spectrum generation, and crystal-axis change from conventional Z-axis-grown PPMgLN to X-axis-grown PPMgLN can help to improve a beam-quality characteristics in the LA-PPMgLN applications.
Acknowledgments
This research was partially supported by Grant-in-Aid for Scientific Research (C) 25390102 by JSPS, and Photon-Frontier-Consortium Project by MEXT of Japan.
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